Frontend: UI for document generation

[syntakker]

The UI should provide:

• selection of a template from a workflow-specific list
• identification of placeholders used in the template not available from the SORMAS context (e.g. Zimmer-Nr. Bearbeiter)
• the possibility to provide values for these extra placeholders
• persistence of these extra values in Localstorage client-side
• generating a document from the selected template and key/value pairs for placeholders using the Service from issue Backend: Template Engine Service #8
• download of the generated document